[all-commits] [llvm/llvm-project] 8f95e9: [unroll-runtime] Relax two profitability limitatio...

[Philip Reames via All-commits]

  Branch: refs/heads/main
  Home:   https://github.com/llvm/llvm-project
  Commit: 8f95e915cd48a04d720935e3b13c3eef116aeb57
      https://github.com/llvm/llvm-project/commit/8f95e915cd48a04d720935e3b13c3eef116aeb57
  Author: Philip Reames <listmail at philipreames.com>
  Date:   2021-11-15 (Mon, 15 Nov 2021)

  Changed paths:
    M llvm/lib/Transforms/Utils/LoopUnrollRuntime.cpp

  Log Message:
  -----------
  [unroll-runtime] Relax two profitability limitations on multi-exit unrolling

This change is mostly about getting rid of some "uninteresting" cases in a follow on deeper heuristic change.  If anyone sees actually interesting code differences out of this, please let me know.  I'm not expecting this to have much impact at all.

Case 1 - With the single deoptimize non-latch exit, we can't have two exiting blocks sharing an exit block.  We can only hit this with a poorly documented debug flag.

Case 2 - Why should we treat epilog cases differently from prolog cases?  Or to say it differently, why should starting with a constant control whether a multiple exit loop gets unrolled?

Sorry for the lack of tests here.  These are both *exceedingly* narrow cases in practice, and after a while trying, I couldn't come up with a test which did anything "useful" as opposed to simply exercise a random combination of force flags.  Note that the legality cases for each are already exercised with force flags.